2010 Grid-Direct String Inverter Guide

Less than a decade ago, the first reliable and efficient high voltage grid-direct string inverters became available in North America. Today, integrators have access to more than 50 listed string inverters from eight different manufacturers.

With so many string inverters to choose from, one goal for this article is to provide a ready resource for you to reference when specifying, selecting or installing these products. The main tool for this purpose is the comprehensive “2010 Grid-Direct String Inverter Specifications” table on pages 44–47. Updated and expanded since the last string inverter guide, published in the October/November 2008 issue of SolarPro magazine, the 2010 table includes next generation products from Fronius USA and KACO new energy, as well as even more data that is useful for PV system engineers, designers and installers. For example, the table now includes acceptable ac and dc cable sizes and terminal torque specifications for each product. Not only is this information available here at your fingertips, but readers who access this article online at solarprofessional.com will also find the same data available for download as an Excel spreadsheet.

Another goal is to help PV system designers and installers get the most out of their string inverters. The first concern, of course, is that these products are used in ways that are safe and Code compliant. Beyond that, everyone benefits from efficient and reliable applications. These are measured in both the short term and over the life of the system, in kilowatt-hours generated, system uptime and customer satisfaction. Efficiency also extends to the actual installation process itself. After all, since SMA America introduced the first Sunny Boy inverters this side of the Atlantic in June 2001, improved product features, such as quick mounting brackets and integrated disconnects, have become increasingly common. String inverter manufacturers are constantly innovating, making installations easier, faster and more cost effective.

The fact that manufacturers have been able to increase their products’ value proposition to integrators and end users while continually driving down the cost of these products is one of our industry’s great success stories. Unfortunately, simple and avoidable system design or inverter installation errors can erode project profitability and customer confidence. This article is premised, in part, on the fact that it is easier and less expensive to prevent an error or oversight rather than to fix the problem that results. With that in mind, many common design and installation pitfalls experienced with string inverters are covered, as well as advanced strategies for the optimal use of these products in a variety of applications.

String Inverter Applications

String inverters are used in both single-phase and 3-phase applications. This is true in spite of the fact that until recently string inverters were available exclusively with a single-phase ac output. (See the sidebar, below, for an explanation of how the term string inverter is defined in this article and why.)

Single-phase applications. By far the most common use for grid-direct string inverters is in single-phase applications, which are primarily residential installations. The fact that almost all of the string inverters listed in the companion table will interconnect to a 120/240 V service points to this being the primary market for these products. Subdividing single-phase string inverter applications further, relatively simple single-inverter applications and relatively complex multiple-inverter applications are reasonable distinctions to draw.

One of the biggest market changes since the introduction of string inverters in North America is just how large of a grid-connected PV system can be installed today using one string inverter. The accompanying specifications table shows that North American PV system integrators now have access to products ranging from 700 to 11,400 W per inverter, which are suitable for arrays ranging in size from 500 to 13,000 W. This is a remarkable range of products to choose from. With individual manufacturers offering anywhere from four to nine single-phase string inverter models per product line, matching array capacity to inverter capacity is more convenient now than ever before.

Industry veterans recall the days when installing an 11 kW residential array would have required the installation of four red boxes on the wall, plus all the required BOS components, none of which was inverter integrated at that time. Going back just over a year, residential PV installations larger than about 8 kW in capacity would have required multiple inverters. The current range of products ensures that in most residential applications, designers can at least consider using a single inverter simply based on array and inverter capacity.

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